1. Field of the Invention
This invention relates to an image display system, and more particularly to an liquid crystal image display system whose dependence on the visual angle and contrast are improved. This invention further relates to an image display system which conforms to the visual sense of viewers.
2. Description of the Related Art
Liquid crystal displays are widely used in notebook computers, electronic pocketbooks, car navigators and the like.
Among such liquid crystal displays, a twisted nematic mode liquid crystal displays are most widely used. The twisted nematic mode liquid crystal display generally comprises a pair of glass substrates having transparent electrodes, liquid crystal cells which are disposed between the glass substrates and contain liquid crystal molecules twisted by 90xc2x0 and a pair of polarizing plates which are disposed on the outer sides of the respective glass substrates with their directions perpendicular to each other. Display of images is effected by controlling the electric voltage applied across the substrates, thereby changing the orientation of the liquid crystal molecules so that the output of light passing through the liquid crystal cell changes. That is, when the electric voltage applied is lower than a threshold voltage, the polarization direction of linearly polarized light passing through the light incident side polarizing plate is rotated by 90xc2x0 along twist of the liquid crystal molecules and accordingly the light passing through the light incident side polarizing plate can pass through the light emanating side polarizing plate which is disposed perpendicular to the light incident side polarizing plate, whereby a bright spot is displayed. On the other hand, as the electric voltage becomes higher than the threshold voltage, the major axes of the liquid crystal molecules begin to erect from the middle portion between the electrodes and the optical activity begins to deteriorate. As the electric voltage is further increased, the optical activity is finally nullified, light passing through the light incident side polarizing plate impinges upon the light emanating side polarizing plate without rotated and accordingly cannot pass the light emanating side polarizing plate, whereby a dark spot is displayed.
Generally the liquid crystal displays are divided into a simple matrix and an active matrix according to the system for applying an electric voltage to each picture element. Those most widely used at present are a super twisted nematic mode liquid crystal display of the simple matrix and a thin film transistor liquid crystal display of the active matrix. However there have been known liquid crystal displays of other various modes.
In the conventional liquid crystal image display systems, there have been a problem that the gradation of the image changes depending on the visual angle and the contrast is low. In order to overcome such a problem, there have been proposed a liquid crystal image display system in which a light guide layer and a light dispersing layer are provided on the cells in order to lessen dependence on the visual angle (Japanese Unexamined Patent Publication No. 58(1983)-95378) and a liquid crystal image display system in which parallel light is projected in order to lessen dependence on the visual angle and increase the contrast.
However, practically it is very difficult to completely collimate light and actually light containing therein components directed in multiple directions projected onto the liquid crystal display, which results in deterioration in contrast since images by oblique components are superposed on the image by vertical components.
Further when light from a light source is completely collimated, the efficiency of utilization of light from the light source deteriorates and brightness is lowered.
Further it has been proposed to provide phase-contrast film on the liquid crystal to lessen change in gradation depending on the visual angle. However even this approach cannot completely overcome the problem of dependence on the visual angle.
Further conventional image display systems are not designed taking into account characteristics of human visual response. That is, since the eyes are arranged in a horizontal direction, the visual response in the horizontal direction is higher than that in the vertical direction and humans can recognize in more detail in the horizontal direction. However in the conventional image display systems, the density and the shape of picture elements are the same in the horizontal direction and the vertical direction. Accordingly, there sometimes happens that information displayed in the horizontal direction is poor for the human visual response in the horizontal direction and information displayed in the vertical direction is too much for the human visual response in the vertical direction.
In view of the foregoing observations and description, the primary object of the present invention is to provide a liquid crystal display which is free from dependence on the visual angle and can display a high contrast image.
Another object of the present invention is to provide an image display system which conforms to the visual sense of viewers.
In accordance with a first aspect of the present invention, there is provided a liquid crystal display system comprising a liquid crystal cell system formed by sandwiching liquid crystal between first and second transparent electrode substrates and a light source for projecting substantially collimated light onto the first transparent electrode substrate, an image being viewed from the second transparent electrode substrate side, wherein the improvement comprises that
an optical compensation means is provided on each of the first transparent electrode substrate side and the second transparent electrode substrate side of the liquid crystal cell system, and
a light dispersing layer is provided on the second transparent electrode substrate side of the liquid crystal cell system.
The xe2x80x9csubstantially collimated lightxe2x80x9d may include components which are at a slight angle to the parallel components.
In the liquid crystal image display system in accordance with the first aspect of the present invention, the optical characteristics of the components at a slight angle to the parallel components are corrected to substantially the same as those of the parallel components by virtue of the optical compensation means disposed on the light incident side and the light emanating side of the liquid crystal cell system, whereby a high contrast image can be displayed. Further by virtue of the light dispersing layer provided on the light emanating side of the liquid crystal cell system, the visual angle-dependence is lessened.
The liquid crystal may be of various types such as twisted nematic liquid crystal.
In accordance with a second aspect of the present invention, there is provided a liquid crystal display system comprising a liquid crystal cell system formed by sandwiching liquid crystal between first and second transparent electrode substrates and a light projecting means for projecting light onto the first transparent electrode substrate, an image being viewed from the second transparent electrode substrate side, wherein the improvement comprises that
said light projecting means is disposed on the first transparent electrode substrate side adjacent thereto and comprises a point light source disposed in a position where it is near the first transparent electrode substrate and does not face the first transparent electrode substrate, a collimating optical system which collimates light emitted from the point light source to parallel light travelling in parallel to the first transparent electrode substrate and a reflecting mirror which is disposed facing the first transparent electrode substrate and reflects the parallel light to impinge upon the first transparent electrode substrate in perpendicular thereto, and
a light dispersing layer is provided on the second transparent electrode substrate side of the liquid crystal cell system.
Said xe2x80x9cposition where the point light source is near the first transparent electrode substrate and does not face the first transparent electrode substratexe2x80x9d is for example a position beside the reflecting mirror which is disposed facing the first transparent electrode substrate.
The collimating optical system may comprise, for instance, a Fresnel lens disposed in perpendicular to the first transparent electrode substrate near thereto and a condenser lens which directs light emitted from the point light source toward the Fresnel lens.
An optical compensation means may be provided on the first transparent electrode substrate side of the liquid crystal cell system with another optical compensation means provided on the second transparent electrode substrate side of the liquid crystal cell system between the light dispersing layer and the liquid crystal cell system.
The liquid crystal cell system may be divided into a plurality of regions and a light projecting means may be provided for each region. That is, a plurality of light projecting means may be provided for one liquid crystal panel. In this case, two or more point light sources may share one reflecting mirror.
The liquid crystal may be of various types such as twisted nematic liquid crystal.
In accordance with the second aspect of the present invention, by collimating light emitted from the point source by the optical system, the light projected onto the first transparent electrode substrate can be parallel light containing less oblique components. Further the efficiency of utilization of light from the light source can be high, whereby power consumption can be reduced.
Further since the light projecting means comprises a point light source which emits light in parallel to the substrates and a reflecting mirror which reflects the light in perpendicular to the substrates, the light projecting means can be small in thickness.
By causing light containing less oblique components to impinge upon the liquid crystal cell system, a high contrast image can be displayed. Further by virtue of the light dispersing layer provided on the light emanating side of the liquid crystal cell system, the visual angle-dependence is lessened.
In accordance with a third aspect of the present invention, there is provided an image display system in which an image signal is reproduced as a visual image on a pixelized screen having a number of picture elements arranged in horizontal and vertical directions, wherein the improvement comprises that the density of the picture elements in the horizontal direction is higher than that in the vertical direction.
It is preferred that the dimension in the vertical direction of each picture element be larger than that in the horizontal direction.
The image signal may be one on which picture element density conversion processing for causing the density of the picture elements in the horizontal direction to be higher than that in the vertical direction has been carried out, one read out in such a manner that the density of the picture elements in the horizontal direction becomes higher than that in the vertical direction, one read out on the basis of picture elements whose dimensions are larger in the vertical direction than in the horizontal direction, or one read out on the basis of picture elements whose dimensions are larger in the vertical direction than in the horizontal direction and at the same time whose density is higher in the horizontal direction than in the vertical direction.
Preferably the density of the picture elements in the horizontal direction is at least 1.2 times as high as that in the vertical direction and more preferably three times as high as that in the vertical direction.
Preferably the dimension of each picture element in the vertical direction is at least 1.2 times as large as that in the horizontal direction and more preferably three times as large as that in the horizontal direction.
The image display system may be of any type including those using liquid crystal, CRT, FED or EL.
It is preferred that a maximum brightness exceeds 800 nit.
When the density of the picture elements in the horizontal direction is higher than that in the vertical direction and the dimension in the vertical direction of each picture element is larger than that in the horizontal direction, the response in the horizontal direction in a high frequency range becomes higher and display conforming to the visual sense of viewers can be achieved.